Machine to machine (referred to as M2M; also called MTC, Machine Type Communication) user equipment (referred to as UE), also known as M2M user communication device, is a main application form of Internet of things at the current stage. Low power consumption and low cost are important guarantees for its large-scale applications. Currently, the M2M technology has been supported by internationally renowned manufacturers such as NEC, HP, CA, Intel, IBM and AT&T and recognized by mobile operators in countries. The M2M devices currently deployed in the market are mainly based on the Global System of Mobile Communication (GSM) system. In recent years, due to the high spectrum efficiency of the long-term evolution (LTE), more and more mobile carriers select the LTE as the evolution direction of the future broadband wireless communication system. Various LTE-based M2M data services will be more attractive. Only the cost of the LTE-M2M devices is lower than that of the MTC user equipment in the GSM system, can the M2M service really go from the GSM to the LTE system.
What mainly affect the cost of the MTC UE lies in the baseband processing and radio frequency. And reducing the transmission and reception bandwidth is a very effective way to reduce the MTC UE cost. That is, the transmission and reception bandwidth of the MTC UE is less than the maximum transmission and reception bandwidth 20 MHz supported on a single carrier of the conventional LTE user equipment (Ordinary Legacy R8/9/10 UE, referred to as OL UE). The reception and transmission bandwidth of the MTC UE can be set to a small bandwidth such as 1.4 MHz or 3 MHz or 5 MHz supported by the LTE system, or new carriers are added for the access. As shown in FIG. 1, both the small-bandwidth MTC UE and the conventional UE can access a multi-carrier LTE system.
Like an OLT UE randomly accessing the LTE system, the MTC UE also performs an initial LTE network access via a Physical Random Access Channel (referred to as PRACH) to achieve uplink timing synchronization. Once the user completes the synchronization, a Radio Resource Control (referred to as RRC) connection scheduling request approval of the eNodeB can be obtained. There are two random access modes, contention based access and contention-free access. All possible random accesses can adopt the contention based random access mode.
For a PRACH in the LTE system, one random access channel corresponds to one random access preamble. The preamble comprises a cyclic prefix (referred to as CP) and a group of preamble sequences. The random access preamble has four formats, whose corresponding parameter values are shown in Table 1.
TABLE 1Parameter values corresponding torandom access preamble formatsPreamble formatTCPTSEQ03168 · Ts24576 · Ts121024 · Ts 24576 · Ts26240 · Ts2 · 24576 · Ts321024 · Ts 2 · 24576 · Ts4 (only for frame structure 448 · Ts 4096 · Tsof the TDD mode)
TCP in the abovementioned Table 1 indicates the length of CP, TSEQ indicates the length of sequence and the value of Ts is Ts=1/(15000×2048) seconds. Preamble format 0 is transmitted in an ordinary uplink subframe; preamble formats 1 and 2 are transmitted in two ordinary uplink subframes; preamble format 3 is transmitted in three ordinary uplink subframes; preamble format 4 can only be transmitted in uplink pilot time slot (UpPTS) in the time division duplex (TDD) mode.
In the frequency domain, one random access preamble occupies the bandwidth, i.e. 1.08 MHz, corresponding to six resource blocks (RBs). The PRACHs with the same time domain position are distinguished in the frequency domain. If they have the same time and frequency positions, they are distinguished through the preamble sequence sent by the UE.
The PRACH has a plurality of time and frequency position configuration schemes, which need to be obtained by looking up the table according to the parameters prach-ConfigurationIndex indicated by the high level. Frequency division duplex (FDD) and TDD have different time and frequency structures. The FDD preamble formats 0-3 have 64 kinds of random access frame structure configurations, and each configuration corresponds to a preamble format, a system frame number and a subframe number accessible by each frame.
There are also 64 kinds of PRACH configurations allowed by the frame structures of the TDD preamble formats 0-4. Each configuration index corresponds to a combination of a determined preamble format, a PRACH density value DRA and a version index rRA. For the TDD, each subframe may have a plurality of random access resources, depending on the UL/DL configuration. For a determined PRACH density value, the time-frequency physical resources needed by different random accesses are indicated with a group of four symbols (fRA, tRA0, tRA1, tRA2). Wherein, fRA is the frequency resource index in a certain time interval; tRA0=0, 1, 2 respectively indicate that the random access resources are present in all the radio frames, the even-numbered radio frames, or the odd-numbered radio frames; tRA1=0, 1 respectively indicate that the random access resources are located in the first half frame or the second half frame; tRA2 is the uplink subframe number where the preamble starts.
In the LTE, the PRACH resource configuration is cell-specific. For a small-bandwidth system, the cell load is small, then a relatively long random access transmission cycle can be used, for a large-bandwidth system, the cell load is relatively large, then a relatively short transmission cycle can be used. The PRACH time-frequency resources are semi-statically distributed within a physical uplink share channel (referred to as PUSCH) and repeated periodically, as shown in FIG. 2.
Prior to the random access, the OL UE can obtain the random access channel configuration information of the access cell through a broadcast control channel (referred to as BCCH). The information comprises: downlink carrier bandwidth, the number of PRACHs, frequency domain position, time domain cycle, preamble formats of the present cell, the number of preamble ZC root sequences and their serial numbers, power, the maximum number of preamble retransmissions, size of response window, contention resolution timing, and so on. The random access preamble sequence uses a ZC (Zadoff-Chu) sequence with a zero correlation zone, and each cell has 64 kinds of available preamble sequences, which are generated by cyclically shifting one or more of root ZC sequences.
The OL UE may select one of the access time slots, and randomly select one of the 64 kinds of available preambles to access randomly. If a plurality of UEs sends the same preamble in the same time-frequency resources, there will be a contention, and a subsequent contention resolution scheme is needed. The PRACH resources selected by the UE imply the Random Access-Radio Network Temporary Identifier (RA-RNTI). The RA-RNTI is related to the frequency domain position at which the UE receives and monitors a random access response and the reception discrimination.
Some problems, for example, how to configure the PRACH resources, will occur when a low-cost and bandwidth-limited MTC UE randomly accesses the LTE system. Furthermore, since the physical downlink control channel (referred to as PDCCH) is whole bandwidth interleaving, and the bandwidth-limited MTC UE may not be able to receive all of the PDCCH control information sent by the large-bandwidth of system, the random access response decoding is difficult, and the success of random access is seriously affected.
The control information transmitted by the enhanced PDCCH (referred to as ePDCCH) comprise the same content as the original PDCCH, and the ePDCCH is located within the PDSCH region, occupying bandwidth resource of one RB, and multiplexing with the PDSCH via the frequency division mode. The bandwidth-limited MTC UE can receive the ePDCCH as well as all information of the PDSCH indicated by the ePDCCH. Moreover, only the R11 UE and the bandwidth-limited MTC UE can identify the ePDCCH, while the conventional OL UE cannot.
There is no effective solution proposed yet for problems of the low-cost bandwidth-limited MTC UE randomly accessing the LTE system.